1. Field of the Invention
The present invention relates to image forming apparatuses such as a copying machine.
2. Description of the Prior Art
There has been conventionally known a copying machine for measuring the original density by prescanning and controlling, for example, the exposure value of a lamp for exposure on the basis of its measured value so as to do suitable printing irrespective of the original density.
The measurement of the original density is made by irradiating the lamp for exposure into an original and detecting the amount of its reflected light by a photoelectric converting device such as a photodiode at the time of prescanning.
FIG. 6 shows the output characteristics of an original density detecting device having a photodiode and an amplifier circuit for amplifying its output.
As obvious from FIG. 6, the rate of change in output of the original density detecting device is large in the range of a relatively low original density, while being small in the range of a relatively high original density.
Thus, the rate of change in output of the original density detecting device is small in the range of a relatively high original density. Accordingly, when the original density is relatively high, it is difficult to measure the precise original density.
In order to increase the rate of change in output of the original density detecting device in the range of a relatively high original density, it is considered that the illuminance of the lamp for exposure at the time of prescanning is made high. By doing so, however, the rate of change in output of the original density detecting device in the range of a relatively low original density becomes small, as shown in FIG. 7. Accordingly, when the original density is relatively low, it is difficult to measure the precise original density.